Para-xylylene polymers are employed as coatings for various electronic components due to their desirable physical and electrical properties. One advantage of poly-para-xylylene coatings is that the thin layers of such coatings are capable of exhibiting highly desirable physical and electrical properties. Because para-xylylene coatings are applied in very thin layers, heat tends to dissipate rapidly from the underlying components. Thus, the coated components cool down quickly and are less prone to temperature related degradation than similar components bearing other types of coating.
In further contrast to conventional polymer coatings, para-xylylenes are generally not prepolymerized prior to application on the coatable substrate. This is so because the para-xylylene polymers are not given to simple extrusion, melting or molding as are many of the conventional thermoplastics. Additionally, because the para-xylylene are generally insoluble in commonly used organic solvents, it is impractical to imply traditional solvent deposition techniques for applying poly-paraxylylene coatings.
Accordingly, in most commercial applications, paraxylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the "parylene process." Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400.degree. to 750.degree. C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film.
Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately -70.degree. C.
The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections.
In typical parylene deposition systems, the cold trap comprises a generally cylindrical containment vessel which is fluidly connected to the deposition chamber via a vapor inlet line, and fluidly connected to a vacuum pump via a vapor outlet line. The vacuum pump, when activated, maintains the system under constant negative pressure. Positioned within the containment vessel is a hollow cooling member which is adapted to be filled with a material such as liquid nitrogen. The cooling member is sized and configured such that when positioned within the containment vessel, an annular space is defined between a portion of the outer surface thereof and the inner surface of the containment vessel.
In prior art cold traps, the vapor inlet line of the containment vessel is oriented such that the monomer vapor which fails to condense within the deposition chamber flows radially into the containment vessel, and in particular the annular space defined between the inner surface thereof and the outer surface of the cooling member. Similarly, the vapor outlet line of the containment vessel is oriented such that air and any residual monomer vapor flows radially out of the annular space. Since monomer vapor will condense and polymerize on any reduced temperature object with which it comes in contact, the radial flow of residual monomer vapor into the cold trap, and hence the direct impingement of the vapor flow against the outer surface of the cooling member, causes parylene to "plate out" and build-up on the outer surface of the cooling member immediately adjacent the vapor inlet line of the containment vessel. Over time, such build-up has the effect of insulating a portion of the cooling member, thus causing less residual monomer vapor to be removed from the air/vapor mixture circulating through the cold trap. Additionally, in extreme cases, the build-up partially or completely clogs the vapor inlet line, thus preventing air/vapor flow into the cold trap which results in an improper coating process and/or damage to the deposition system.
Due to the inability of prior art cold traps to substantially remove residual monomer vapor from the air/vapor mixture circulating therethrough, the deposition systems incorporating the prior art cold traps are typically provided with a filter which is disposed within the outlet line intermediate the containment vessel and the vacuum pump. Since the previously described build-up of parylene diminishes the ability of the cold trap to effectively remove residual monomer vapor from the air/vapor mixture circulating therethrough, the filter is provided to prevent the vacuum pump from being exposed to any residual monomer vapor, and to further prevent any residual monomer vapor from being exhausted by the vacuum pump, and hence the deposition system, into the surrounding environment.
Though the filter functions to reduce the residual monomer vapor levels in the air/vapor mixture flowing therethrough, a small amount of monomer vapor usually still remains within the air stream entering the vacuum pump, particularly when the removal capacity of the cold trap is lessened by the previously described build-up of parylene on the cooling member. Since the condensation deposition of coatings is not substrate selective (i.e., the vapors have no way of seeking out only the desired substrates), the monomer vapor entering the vacuum pump condenses and polymerizes on any reduced temperature internal component of the pump with which it comes in contact. Such coating of the internal pump components necessitates the frequent cleaning thereof, and often results in pump failure thus requiring time consuming repair or the more expensive alternative of complete replacement. Additionally, a small amount of residual monomer vapor may be exhausted from the pump which could pose a health hazard. In addition to the vacuum pump requiring frequent cleaning, the build-up of parylene within the cold trap also necessitates the frequent and time-consuming cleaning of the cold trap, and in particular the outer surface of the cooling member. The present invention overcomes these and other deficiencies associated with prior art cold traps by providing a tangential flow cold trap which increases the effectiveness of the cold trap by allowing greater contact of the monomer vapor with the internal surfaces thereof, thus reducing the maintenance levels associated with the cold trap as well as the vacuum pump.